Ordinal outcome support

[ck37]

Hello,

Thank you for an excellent package and method. Is there any potential interest in supporting ordinal outcomes with causal forest? I couldn't find anything in the docs or issues thus far, and would be happy contribute a little time to make that happen if it's of interest.

I did find this ordinal version of random forest that could be helpful to review: https://link.springer.com/article/10.1007/s00357-018-9302-x

Appreciate it, Chris